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For decades, scientists were at a loss to explain the source of
the so-called Magellanic Stream, a long ribbon of gas discovered
in the early 1970s that extends nearly halfway around the Milky
Way.

But new data from NASA's
Hubble Space Telescope have helped astronomers crack the
case. The observations show that the stream did not form all at
once; instead, the ribbon is a combination of material stripped
at different times from both the Large Magellanic Cloud and
the Small Magellanic Cloud, two satellite galaxies that hover
around the Milky Way less than 200,000 light years away.

The team determined the source of the ribbon by measuring the
abundances of heavy elements at six locations different along the
Magellanic Stream using Hubble's Cosmic Origins Spectrograph,
according to a statement from NASA. (Scientists can detect theses
faraway heavy elements through the way they absorb ultraviolet
light.)

Low levels of oxygen and sulfur found throughout most of the
stream match the chemical makeup of the Small Magellanic Cloud,
the researchers said. But they also found a surprising amount of
sulfur in a part of the stream that's closer to the
Magellanic Clouds.

"We're finding a consistent amount of heavy elements in the
stream until we get very close to the Magellanic Clouds, and then
the heavy element levels go up," study researcher Andrew Fox of
the Space Telescope Science Institute in Baltimore, Md., said in
a statement. "This inner region is very similar in composition to
the Large Magellanic Cloud, suggesting it was ripped out of that
galaxy more recently."

By studying the stream, researchers could learn more about how
larger galaxies affect their surrounding satellite galaxies, Fox
explained.

"We want to understand how galaxies like the Milky Way
strip the gas from small galaxies that fall into them and then
use it to form new stars," he added in a statement. "This seems
like it's an episodic process. It's not a smooth process where a
slow stream of gas comes in continuously. Instead, once in a
while a large gas cloud falls in. We have a way of testing that
here, where two galaxies are coming in. We've shown which of them
is producing the gas that ultimately will fall into the Milky
Way."

The findings were detailed in two papers that appeared in the
Aug. 1 issue of The Astrophysical Journal.